The invention lies in the field of semiconductor technology. Specifically, the invention pertains to a method of fabricating a micromechanical semiconductor configuration with a thin membrane formed inside a cavity.
A micromechanical semiconductor configuration of that type is utilized, for example, as a semiconductor acceleration sensor for measuring accelerations. In that use the membrane together with counter-electrodes serves as a capacitor. Changes in the capacitance of the device are evaluated as the measured parameter. The membranes in the prior art are normally fixed through springs in the sensor. However, the process sequences during fabrication of the membrane lead to stress, especially mechanical stress in the membrane. In the event that relaxation through the springs is incomplete the membrane can be permanently deformed. In addition, forces taken up in the springs can lead to faulty behavior of the membrane during operation. In order to protect and mechanically stabilize these sensors, upper sealing plates, generally consisting of a polysilicon layer, are braced by means of nitride supports, for example. The sealing plates can also be formed by metallization and be supported with metallization supports. In each case, additional layers are necessary for the formation of the sensor.
It is accordingly an object of the invention to provide a method of fabricating a micromechanical semiconductor configuration, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be fabricated more simply and therefore more economically and which, at the same time, achieves improved mechanical and physical characteristics.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of fabricating a micromechanical semiconductor configuration, in particular a microelectronic integrated sensor device. The method comprises the following steps: forming an epitaxial layer sequence of individual crystalline layers on a substrate, the layer sequence including a support layer, a membrane layer, and a counter-support layer, and producing a doping profile with alternating or varied doping of the support layer, the membrane layer and the counter-support layer;
forming a membrane from the membrane layer in the layer sequence by at least regionally wet-chemically etching the support layer and the counter-support layer with an etching agent selective towards the membrane layer, and thereby forming a cavity in the layer sequence whereby an edge region of the membrane rests on a support formed by the support layer, and the membrane is covered with a covering layer supported on the counter-support layer.
According to an essential idea of the invention the membrane is formed through a crystalline layer within the substrate or within an epitaxial sequence off layers of the semiconductor configuration formed on a substrate. Thus fabrication of the micromechanical semiconductor configuration does not involve the use of any layers that are different from the layers normally used in the fabrication of semiconductors. Rather, the standard process used for conventional semiconductor components can also be taken over for the fabrication of a membrane in a cavity of the micromechanical semiconductor configuration, whereby simply minor process steps need to be inserted or amended.
The entire micromechanical semiconductor configuration is advantageously fabricated from a single combined crystalline semiconductor base material which can be either the actual substrate or an epitaxial series of layers arranged on a substrate.
In accordance with an added feature of the invention, the membrane layer, and thus the membrane, is formed of mono-crystalline silicon. In addition to the economical fabrication costs, the essential advantages lie above all in the use of an almost stress-free monocrystalline layer for the membrane and thus in an improvement in the mechanical and other physical properties.
In accordance with an additional feature of the invention, the membrane is doped differently from the support layer and the counter-support layer.
In a preferred implementation the membrane is laid at the edge region on a support and covered with a covering layer supported on the counter-support. The counter-support and covering layer hereby also function as lateral and upper limits of movement which are so arranged relative to the edge of the membrane that the membrane can move to compensate mechanical stress.
In an especially preferred embodiment of the invention the support, the membrane, the counter-support and the covering layer in that order are each fabricated through a crystalline layer within the substrate or within an epitaxial series of layers of the semiconductor configuration arranged on the substrate, whereby a doping profile is set up in the substrate or in the epitaxial series of layers such that at least the support and the counter-support, on the one hand, and the membrane, on the other, are sufficiently differently doped that in order to fabricate a cavity the layers surrounding the membrane can be wet chemically etched by means of a suitable selective etching solution. The doping profile can be set up either by subsequently performing one or several high-energy implantation steps, or already with the deposition of differently doped epitaxial layers. In a preferred embodiment the sequential layers of the substrate or the series of epitaxial layers are doped alternately high and low. Depending on the etching solution used, it is then possible through a subsequent wet chemical etching process to remove either the highly doped regions, for example using an HFxe2x80x94HNO3xe2x80x94CH3COOH etching solution, or to remove the low doped regions, for example by means of a KOH etching solution. This procedure exploits the selectivity of the respective wet chemical etching process between layers with high-level doping and layers with low-level doping, whereby selectivities of approximately 50:1 can be achieved.
In addition, the sequential areas in the substrate or in the epitaxial series of layers can be alternately doped between p-doping and n-doping, which alternation offers the additional advantage of electrical separation of adjacent layers.
In accordance with another feature of the invention, the membrane is formed with etching holes. It is useful to provide etching holes in the membrane initially covering the supporting layer in order to enable the etching solution to penetrate into the layer lying below. For the same purpose the covering layer can also be provided with appropriate etching holes, whereby in order to form a completely closed cavity the holes are closed in a later process step, preferably by means of a free-flowing glass layer such as BPSG or similar.
In accordance with a further feature of the invention, the membrane is supported between the support and a counter-support formed by the counter-support layer.
In accordance with a concomitant feature of the invention, the support and/or the counter-support are formed to support an entire peripheral region of the membrane.
The micromechanical semiconductor configuration can be successfully employed in all areas where there is a requirement for micromechanical structures with membranes in cavities. In particular, the micromechanical semiconductor configuration according to the invention can be used as a semiconductor acceleration sensor or as a semiconductor micropump.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a micromechanical semiconductor configuration and method for fabrication of a micromechanical semiconductor configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.